The present invention relates to amplifiers for amplifying electric signals and, more particularly, to amplifiers for amplifying the frequency dependent output of transducers such as phonograph and magnetic tape playback transducers.
Known transducers for providing audio frequency range electrical signals from a recording media, such as a phonographic record or a magnetic media, characteristically provide a voltage output that varies with frequency. Typical phonograph transducers of the magnetic type, such as the moving magnet/iron or the moving coil type, have a frequency dependent transfer function that exhibits a 40 dB increase in output voltage with increasing frequency across the audio frequency spectrum and oftentimes exhibit reasonant `peaking` at one or more regions within the audio frequency spectrum. Similar frequency dependent transfer characteristics are also associated with playback heads used to obtain electrical signals from a moving magnetic media, such as magnetic tape, in which the playback head output variation approaches 50 dB.
Because of the known variation in transducer output with frequency, industry standards have been established by the RIAA and the NAB for phonograph and magnetic media preamplifiers so that the preamplifier output compensates for the frequency dependent output of the transducer and the characteristics of the recording media to provide a reasonably flat output over the audio frequency spectrum.
Past techniques for implementing the RIAA, the NAB, or other standard characteristic curves have involved the use of a frequency dependent RC network functioning as a passive filter in the forward signal path or as part of a frequency dependent negative feedback path.
In the passive approach, an RC or equivalent device network is inserted into the forward signal path, typically between two amplifier stages. The first stage amplifies the transducer output and provides this initially amplified signal to the RC network which is configured to have a frequency dependent response that is the inverse of the transducer's so that the output of the RC network is substantially flat over the audio frequency spectrum. The second stage then amplifies the now-flat signal to compensate for the insertion loss of the RC network and provide a signal that is compatible with the next succeeding stage of amplification or other signal utilizing device.
While the passive equalization approach is generally satisfactory, there are a number of attendant disadvantages. The first stage of amplification must have a fairly wide dynamic range since it must provide adequate gain at the lower frequency portion of the audio frequency spectrum where the transducer output is relatively low and still effect adequate amplification at the high end of the audio frequency range where the transducer output is 40-50 dB higher for the same program material level. If the dynamic range of the first amplification stage is insufficient, undesirable amplifier saturation can occur at the high end of the audio frequency spectrum. Since typical RC equalization networks have a substantial insertion loss, the post-network amplifier stage must have high enough gain to compensate for the insertion loss although high-gain signal amplification increases the probability of a noise component being impressed on the amplified signal.
In the active equalization approach, a frequency dependent RC network is inserted into the degenerative or negative feedback path of the amplifier so that the amount of feedback and the consequent forward gain, A, of the amplifier is frequency dependent so as to have an essentially flat output over the audio frequency spectrum.
The active equalization approach, while effective for its intended purpose, also has a number of attendant drawbacks. Since the forward gain, A, of the amplifier must be varied over a wide dynamic range by varying the magnitude of the negative feedback, sonic `coloring` can be introduced into the signal at the lower and higher frequency ends of the audio frequency spectrum. At the lower end of the spectrum, where high gain is required, the amplifier can exhibit insufficient closure ratio, inadequate distortion reduction, and high output impedance. At the higher frequency end of the audio frequency spectrum, where 40-50 dB less gain is required, the increased amount of negative feedback necessary to achieve the desired gain reduction can lead to instabilities, phasing problems, and transient induced distortions in the amplified signal.
While each of the above described equalization approaches can provide amplifiers having comparable steady state quantitative performance, each approach has attendant drawbacks that can undesirably color the qualitative or sonic performance of the amplifier, especially at the lower and upper ends of the audio frequency spectrum.